An established clinical characteristic variable is the Fractional Flow Reserve (FFR). The FFR may be measured with a pressure wire for example. The pressure wire is guided past a stenosis in the body vessel or body vessel segment and determines the pressure there distal to the stenosis. The distal pressure is divided by the proximal pressure in order to calculate the fractional flow reserve.
With a three-dimensional model of the body vessel segment or body vessel section in which the stenosis is contained, and further boundary conditions, such as for example the blood flow in milliliters per second through the body vessel segment, the pressure curve via the stenosis may be calculated using mathematical methods of fluid dynamics (computational fluid dynamics). A virtual value may be computed for the fractional flow reserve, a virtual FFR value, virtually on the basis of the three-dimensional model. Methods are described, for example, in the article by Paul D. MORRIS et al.: ““Virtual” (Computed) Fractional Flow Reserve—Current Challenges and Limitations” in JACC: Cardiovascular Interventions, Vol. 8, No. 8, 2015, pages 1009 to 1117. Other methods of computation for a virtual FFR value are also known.
The approaches to virtual computation of the fractional flow reserve may be divided up into two groups. Non-invasive methods, in which geometry information about the body vessel segment or body vessel is obtained by computed tomography, magnetic resonance tomography or other methods, and minimally invasive methods, in which the geometry information is obtained in the cardiac catheter laboratory by an injection of contrast medium into the vessel with a subsequent x-ray recording. A non-invasive examination of a patient is initially undertaken by computed tomography (CT). As well as the diagnostic information about one or more vessel cross sections of the examined body vessel segment or body vessel, a virtual value for the fractional flow reserve may also be computed that is referred to as the CT FFR value. By contrast, a virtual value for a fractional flow reserve, that is established by an angiography in the cardiac catheter laboratory for example, will be referred to as an angio FFR value.
The CT FFR method, e.g. the computation of the virtual FFR value by CT, includes a three-dimensional model of the entire vascular tree in which the body vessel or the body vessel segment with the stenosis is located, is available. The CT FFR method allows for a good determination of the perfused myocardial mass as well as of the perfusion flow derived from the proportion of the perfused myocardial mass. Furthermore additional information, such as for example a composition of the stenosis or of the plaque, may be established. The disadvantage is the comparatively low spatial resolution and thus an imprecise geometry representation of the stenosis geometry.
By comparison, the angio FFR method, e.g. the computation of a virtual FFR value using an angiography, includes good spatial resolution, that makes a precise representation of the stenosis geometry possible. A disadvantage is the estimation of the blood flow via the vessel cross sections. Even small errors may result in large effects. The estimation of the blood flow via contrast media dynamics is complex and difficult in the angio FFR method. A further disadvantage is that the angio FFR method does not deliver any information about a state of the myocardial mass, that is important for example for recognizing any possible prior damage and enabling it to be taken into account during a treatment. Geometry information of the vascular tree as a whole may only be obtained with great difficulty, that is also attributable to the relatively small detectors used in angiographies.